to buy as much of a desired product as one can afford with the amount of money one is carrying.

Cashier: Can I help you?
Customer: I’ll take some of those one dollar scratch-off tickets right there.
Cashier: Sure, how many would you like?
Customer: A Cowboy's Dozen (throws money on counter)
Cashier: (counts money) Soooo, seven then?
Customer: I reckon.
Cashier: Good luck!

Could also be used to exaggerate a story:
Derek: How much money did he say he won on that bet?
Jeff: A Cowboy’s $300.
Derek: Meaning?
Jeff: $200